Originally Posted by
Seattle Forrest
What do you really mean by max? Is it the highest heart rate you can achieve at your current fitness level? The most your body will ever be able to achieve? What you're up to today, after staying up too late and making up for it with lots of coffee? It's a slightly fuzzy concept, which leads to slightly different answers if you measure on different days. Keep in mind 170 vs 173 is within 2 %, the margin of error is bigger than that for optical HRMs.
I would take the max to mean the highest heart rate you can achieve at your current fitness level. I wonder whether the OP's max increased or whether he or she pushed to slightly higher values (possibly not maximal) on the three occasions mentioned. It's really hard to push yourself to your true max.